The present application claims priority to Japanese Patent Application No. 2002-026842 filed in the Japanese Patent Office on Feb. 4, 2002 and Japanese Patent Application No. 2003-000118 filed in the Japanese Patent Office on Jan. 6, 2003, the disclosures of which are hereby incorporated by reference herein in their entirety.
1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, and more particularly to a developer carrier in the developing device that carries a developer thereon to develop a latent image formed on an image carrier.
2. Discussion of the Background
In a background developing device that develops a latent image formed on an image carrier with a developer in an image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus that forms images at a high speed or a middle speed, a surface of a developing sleeve as an example of a developer carrier is subjected to a sandblast treatment or a groove treatment to impart an appropriate surface roughness. Such a treatment is performed to prevent the decrease of image density caused by the developer that slips and remains on the developing sleeve rotating at a high speed.
In the case of the sandblast treatment, materials of a developing sleeve can be aluminum, brass, stainless, conductive resin, etc. In view of cost and accuracy in shape, aluminum is generally used as the material of the developing sleeve. When a surface of a developing sleeve made of aluminum is subjected to a sandblast treatment, concave/convex portions are formed on the surface of the developing sleeve by spraying abrasive grains on the surface of a cold aluminum tube in a shape of sleeve which has been extruded at a high temperature. The surface roughness of the developing sleeve is generally in a range of about 5 xcexcm to 15 xcexcm in a ten point mean surface roughness (Rz) scale, which is prescribed in JIS (Japanese Industrial Standards). In the developing sleeve subjected to a sandblast treatment, even though the developing sleeve rotates at a high speed, developer is caught in concave/convex portions formed on the surface of the developing sleeve, and thereby the slip of the developer on the surface of the developing sleeve is obviated.
However, in the developing sleeve subjected to a sandblast treatment, concave/convex portions on the surface of the developing sleeve are abraded with time, thereby deteriorating a developer conveying capability of the developing sleeve. Therefore, a problem of durability of the developing sleeve occurs. Such a problem of durability may be improved by using stainless having high hardness as a material of a developing sleeve or by performing a hardening treatment on a surface of a developing sleeve. However, this results in an increase of cost.
In the case of the groove treatment, materials of a developing sleeve can be aluminum, brass, stainless, conductive resin, etc. In view of cost and accuracy, similarly as in the sandblast treatment, aluminum is generally used as the material of the developing sleeve. When a surface of a developing sleeve made of aluminum is subjected to a groove treatment, an aluminum tube in a shape of sleeve extruded at a high temperature is cooled, and grooves are formed on the surface of the aluminum tube in a shape of sleeve by use of a die. Each of the grooves typically has a cross-section of trapezoid-shape, V-shape, U-shape or the like. The depth of each of the grooves measured from the surface of the developing sleeve is about 0.2 mm. The number of grooves of the developing sleeve having an outer diameter of, for example, 25 mm is typically about 50. In the developing sleeve subjected to a groove treatment, even though the developing sleeve rotates at a high speed, developer is caught in grooves formed on the surface of the developing sleeve, and thereby the slip of the developer on the surface of the developing sleeve is obviated. As compared to the developing sleeve subjected to the sandblast treatment, grooves are not largely abraded even in a long period of use, and the developing sleeve can stably convey the developer.
However, in the developing sleeve subjected to the above-described groove treatment, periodical variations in an image density caused by grooves, that is, an uneven density in a form of a groove pitch typically (hereafter simply referred to as a xe2x80x9cgroove pitch-like uneven densityxe2x80x9d) occurs. Generally, as a depth of groove increases, the developer conveying capability of a developing sleeve enhances, but the groove pitch-like uneven density tends to occur. On the other hand, as a depth of groove decreases, the groove pitch-like uneven density does not tend to occur, but the developer conveying capability of a developing sleeve deteriorates. Especially, recently, as image reproducibility has been improved due to the enhanced image forming technique of development using small-particulate toner and carrier and of development by a developing device in which an image carrier and a developer carrier are provided close to each other, the groove pitch-like uneven density tends to occur.
To prevent occurrence of a groove pitch-like uneven density and to maintain a developer conveying capability of a developing sleeve, the inventor has proposed a developer carrier in which a depth of each of grooves is set in an optimal range. In this proposed developer carrier, a depth of each of grooves is set to be relatively smaller than before, specifically in a range of 0.05 mm to 0.15 mm.
However, when performing an image forming operation by use of the above-described proposed developer carrier, an uneven image density in a relatively long period corresponding to one rotation of the developer carrier (hereafter referred to as a xe2x80x9cperiodic uneven image densityxe2x80x9d) occurred. As a cause of such a periodic uneven image density has been considered to be an eccentricity of the developer carrier, an amount of eccentricity of the developer carrier was measured. However, the measured amount of eccentricity of the developer carrier was not to a degree which causes the periodic uneven image density.
Therefore, it is desirable to provide a developer carrier which has a plurality of grooves on a surface thereof and does not cause the periodic uneven image density.
According to one aspect of the present invention, a developer carrier includes a developer carrying surface configured to carry a developer thereon to develop a latent image formed on an image carrier. The developer carrying surface includes a plurality of grooves to carry the developer. Dispersion D (%) in depth of the plurality of grooves which is calculated according to a following equation is at most approximately 30%:
D(%)={(Axe2x88x92B)/2}/C
where A is a maximum depth of the plurality of grooves, B is a minimum depth of the plurality of grooves, and C is an average depth of the plurality of grooves.
According to another aspect of the present invention, a developing device for developing a latent image formed on an image carrier includes a developer carrier. The developer carrier includes a developer carrying surface that is configured to carry a developer thereon to develop a latent image formed on an image carrier. The developer carrying surface includes a plurality of grooves to carry the developer. Dispersion D (%) in depth of the plurality of grooves which is calculated according to a following equation is at most approximately 30%:
D(%)={(Axe2x88x92B)/2}/C
where A is a maximum depth of the plurality of grooves, B is a minimum depth of the plurality of grooves, and C is an average depth of the plurality of grooves.
According to further aspect of the present invention, an image forming apparatus includes an image carrier configured to carry an image, a latent image forming device configured to form an electrostatic latent image on a surface of the image carrier, and a developing device configured to develop the electrostatic latent image to form a toner image on the image carrier. The developing device includes a developer carrier. The developer carrier has a developer carrying surface configured to carry a developer thereon to develop a latent image formed on an image carrier. The developer carrying surface includes a plurality of grooves to carry the developer. Dispersion D (%) in depth of the plurality of grooves which is calculated according to a following equation is at most approximately 30%:
D(%)={(Axe2x88x92B)/2}/C
where A is a maximum depth of the plurality of grooves, B is a minimum depth of the plurality of grooves, and C is an average depth of the plurality of grooves.
According to yet further aspect of the present invention, a process cartridge for use in an image forming apparatus includes an image carrier configured to carry an image, a developing device configured to develop an electrostatic latent image to form a toner image on the image carrier. The developing device includes a developer carrier having a developer carrying surface. The developer carrying surface is configured to carry a developer thereon to develop a latent image formed on an image carrier. The developer carrying surface includes a plurality of grooves to carry the developer. Dispersion D (%) in depth of the plurality of grooves which is calculated according to a following equation is at most approximately 30%:
D(%)={(Axe2x88x92B)/2}/C
where A is a maximum depth of the plurality of grooves, B is a minimum depth of the plurality of grooves, and C is an average depth of the plurality of grooves.